Removed the resource preview thread from the server build

This solution seems to fix my reproduction case for the hanging server build. The problem seems to start after e8d31cc765d5 but I could not do a proper bisect, I used revert instead but I am not sure about the outcome. I was able to reproduce on multiple Linux systems.

I found out that the export process was stuck in the EditorResourcePreview::stop() function, in the while(!exited) part, so I came up with this "fix" but it seems very dirty.

Comments, opinions and improvements are very very welcome, maybe from @hpvb and @reduz ? Thanks in advance!